When applying minimally invasive techniques in diagnosis and intervention, relying on 2D imaging comes with the disadvantage of a lack of depth perception. The performance and safety of delicate dissection or suturing during minimally invasive procedures may suffer when using 2D imaging, because a surgeon has to rely on motion parallax and other indirect evidence of depth. Where this is not sufficient, he needs to touch the tissues, in order to gauge depth, thereby significantly reducing the speed and precision at which minimally invasive surgical procedures can be performed.
For this reason there have been tremendous efforts of converting 2D data into reconstructed 3D impressions. Angiography, ultrasound, computer tomography, and magnetic resonance imaging are typical examples, where significant progress has been made in this field. Besides the risk of relying on software-generated images, these techniques often do not show real-time performance and lack the precision required for areas such as neurosurgery.
Reflected light images from neural structures disclose several correlates of neural activity (Rector et al., Journal of Neuroscience Methods 91 (1999) 135-145). Depending on the illumination wavelength used, indications of hemodynamic changes (e.g. 560 nm), metabolic protein conformational changes (<400 nm), or neuronal swelling (>600 nm) can serve as measures of local neural activity. Light reflectance techniques typically illuminate the brain surface with monochromatic light, and then detect back-scattered light from the tissue using a charged-coupled device (CCD) camera. Changes in the amount of back-scattered light are calculated as differences or ratios across time on a pixel-by-pixel basis, thereby forming a parametric map of light scattering or absorption changes within the tissue of interest. Such procedures for recording from large neural populations provide insight into neural interactions and neural network properties. The need to study many neurons simultaneously has driven significant advances in optical measurements of neural activation.
Most imaging studies using intrinsic optical signals have employed slow scan imaging technologies and steady-state stimulation to visualize hemodynamic changes associated with neural activation. Although some components of the hemodynamic signals are comparatively fast (<1 s), long integrated signal acquisition is used to average over fluctuations associated with the cardiac cycle. Subtraction techniques isolate those signals associated specifically with neural activity, and tend to eliminate vessel artifacts.
Attempts to image dynamic processes require more sophisticated characterization of optical changes associated with the cardiac cycle. In addition, a number of in vitro studies have identified fast light scattering changes associated with neural swelling during activation that parallel electrical events (Salzberg B M et al., J Gen Physiol 1985; 86:395-411 and Tasaki et al., Biochem Biophys Res Commun 1992; 188(2): 559-564). In order to image such changes in vivo, it is necessary to achieve high sensitivity, high time resolution, and to adequately account for the dynamics of other ongoing physiological processes that affect the overall optical signal. These include movement and spectral changes associated with the cardiac cycle.
Contact of the image probe with the tissue surface can serve as a mechanism for minimizing movement artifacts associated with the cardiac and respiratory cycles. Such contact stabilizes the tissue relative to the probe surface, and reduces pulsations typically seen in these preparations. For chronic studies, the bone is subsequently sealed around the probe using bone wax and dental acrylic allowing restoration of CSF pressure and further stabilizing the brain tissue through hydrostatic forces. Another mechanism for minimizing movement artifact involves an opening in the skull, which is then sealed with a glass window and filled with oil. Such methods minimize brain movement through the establishment of CSF pressure related hydrostatic forces; however, movements in neighboring brain regions (or a nearby vessel) may introduce movements through hydraulic effects. Additionally, use of an oil filled chamber precludes dark-field techniques, and specula reflectance becomes a major issue in the scattered light signals.
Previously in vivo optical techniques using coherent fiber optic image conduit have a focal plane limited to the tissue surface in contact with the probe. Such procedures generally form good images of the tissue surface. Also, since illumination surrounds the imaged area, dark-field methods eliminate specula reflectance from the tissue surface, and provide scattering information from deeper tissue (Rector et al, supra). Unfortunately, light from deeper structures is out of focus and the image is blurred. Because cells of interest are frequently located several hundred microns below the surface, it is desirable to focus below the surface to accurately measure deep structures. Optical techniques with deep focus capabilities would be especially useful for brain structures, which are otherwise difficult to access.
PCT patent application WO 03/050590 A1 describes an endoscopic imaging system. The endoscopic system uses a commercially available endoscopic sheath and uses a microelectro-mechanical (MEM) micromirror to direct the incoming light beam to the lens and onto the sample area of interest. The scanning area of the system is forward-looking and directly ahead of the lens of the endoscope.
However, there remains a need for an imaging device capable of 3-D bio-imaging using minimally invasive techniques for use in dental applications, orthopedic surgery applications and for cancer diagnostic applications such as in providing an optical biopsy, for example. In addition, there is a need for an imaging system capable of focusing below the surface of the tissue to scan deep structures such as neurological structures, for example.